The invention relates generally to heat pump systems and, more particularly, to a method and apparatus for controlling supplemental electric heat during the process of defrosting the outdoor coil thereof.
In conventional heat pump system operation, when the system is operating in the heating mode with the outdoor coil acting as an evaporator, the formation of frost or ice on the heat exchanger surface is a common phenomenon. That is, under appropriate ambient conditions, the media in flowing heat transfer relationship with the evaporator, typically air, has its temperature lowered below its dew point, thus causing condensation to form on the coil. If ambient temperature conditions are sufficiently low, this condensation will then be caused to freeze. That is, since the heat pump operating in the heating mode requires refrigerant to be at a lower temperature than the ambient air in order to transfer heat to the refrigerant by way of the outdoor coil, condensation, and eventually ice or frost, will tend to form on the coil even at ambient temperatures above the freezing point. Once this ice or frost coats the surface of the heat exchanger, the efficiency thereof is impaired, and overall system efficiency and capacity are decreased. In turn, the temperature of the air supplied to the conditioned space will drop, potentially to an uncomfortable level. Consequently, it is desirable to maintain the evaporator surfaces free from ice or frost. A defrost cycle is therefore periodically used as a normal mode of operating for that purpose.
A conventional manner of defrosting the outdoor coil is that of reversing the refrigerant flow, such that the outdoor coil functions as a condenser with the hot gases that are discharged from the compressor being circulated directly to the outdoor coil to melt the ice that is formed thereon. During this process, the indoor coil section acts as an evaporator with the refrigerant removing heat from the air being blown across it. This air is then returned to the conditioned space at a greatly reduced temperature. This undesirable condition is called "cold blow". One method of reducing "cold blow" is to energize electric resistance heater elements in the supply air stream. In many cases, these elements are not sufficient to overcome the cooling capacity of the system and cold air is still introduced to the conditioned space creating an uncomfortable situation for the occupants. In other cases, these elements are greatly oversized and the supply air temperature becomes high enough to satisfy the room thermostat. This can result in an incomplete defrost cycle as well as unnecessary energy use.
It is therefore an object of the present invention to eliminate "cold blow" during the defrost cycle.
Another object of the present invention is to maintain the temperature in the conditioned space at a comfortable temperature during the defrost cycle.
Yet another object of the present invention is the provision for reducing the time required for a defrost cycle.
Yet another object of the present invention is the provision for eliminating the use of excess electric heat during defrost while providing a complete defrosting cycle for the outdoor coil.
Still another object of the present invention is the provision for a heat pump system which is economical to manufacture and economical and effective in use.
These objects and other features and advantages become more readily apparent upon reference to the following description when taken in conjunction with the appended drawings.